This invention relates to novel polymers that have net negatively charged backbones and spiro-ammonium cations. These polymers are particularly useful in heat-sensitive printing plate formulations.
There is current interest in the development of polymers that undergo irreversible thermally driven reactions. Such reactions may result in crosslinking chemistry, changes in solubility, or changes in the surface energy of a polymer film. There is also considerable interest in the minimization of volatile organic compounds in coating compositions. Thus, thermally-sensitive polymers that are water-soluble or water-dispersible are of particular value. The ability of a polymer to undergo such thermally driven changes by means of a reaction in which small molecule byproducts are not emitted is also desirable for many chemical and coating processes in many industries.
U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive.
Low molecular weight (monomeric) spiro-ammonium salts (1-azoniaspiro salts) are known in the field of organic chemistry. Cationic polymers containing spiro-ammonium ions covalently bound within the main chain are also known in the art, having found uses as mordants. See, for example, Muellen et al, Polym. Bull. (Berlin), 1990, 24, 263, De Vynck et al, Macromol. Rapid. Commun, 1997, 18, 149, and U.S. Pat. No. 3,741,768 (Van Paesschen et al).
These compounds, however, are undesirable for several reasons. First of all, the ammonium cations described in the prior art are immobilized within the polymer backbone. This would affect their solubility and usefulness in thermally-sensitive systems. Moreover, it is highly unpredictable as to what interactions or compatibility they might have with other common ingredients in industrial and coating formulations (such as surfactants, colorants, and thickeners). From the perspective of synthetic feasibility, it is typically more difficult to incorporate specific units (in this case, a spiro-quaternary ammonium ring system) into the main chain of a polymer than it is to introduce a desired counterion.
There is a need to provide thermally-sensitive polymers that do not exhibit the noted problems, and that are prepared from common and inexpensive starting materials.
This invention provides an ionomer having carboxylate recurring units in its backbone and an ammonium counterion for the carboxylate recurring units wherein one or more positively charged nitrogen atoms of the counterion are positioned at tetrahedral vertices of one or more spiro bicyclic ring systems.
The polymers (ionomers) of this invention are specifically designed to undergo thermally driven nucleophilic substitution reactions in which the carboxylate moieties ionically bound to the polymer backbone attack the carbon atoms in the xcex1-position in relation to the quaternary ammonium centers of spiro-ammonium counterions. This results in the cleavage of carbon-nitrogen bonds, the formation of ester bonds, and the dequaternization of the ammonium ions. This reaction will result in changes in solubility and surface energy of the polymer. The advantage of the spiro-ammonium counterion lies in the fact that no matter which of the carbon positions (xcex1 to the quaternary ammonium center) are attacked, the tertiary amine product remains bound to the polymer backbone by an ester bond and no volatile compounds are emitted. This reaction is exemplified in Reactive Scheme 1 below.
Reactive Scheme 2 below shows a similar reaction in which a dicationic counterion is used in the polymer. In that situation, the resulting polymer is crosslinked and no volatile compounds are emitted.
The polymers of this invention may find utility in water-based thermally hardenable coating formulations. This class of novel materials is particularly useful for markets and applications having stringent environmental requirements. These polymers are also useful in thermal imaging applications, such as thermally-sensitive printing plates, as described in copending U.S. Ser. No. 09/454,151, filed Dec. 3, 1999 by Leon and Fleming.
It is also an advantage that some polymers of this invention can be derived from such common and inexpensive ethylenically unsaturated polymerizable monomers as acrylic acid and methacrylic acid, and hence can be easily copolymerized with other common monomers using known techniques and reliable methods. 
The polymers of this invention comprise random recurring units at least some of which comprise carboxylic acids groups and the particular spiro-ammonium cations described herein. The polymers generally have a molecular weight of at least 2000 Daltons and preferably of at least 5000 Daltons.
The carboxylate-containing polymers can be chosen or derived from a variety of polymers and copolymer classes including but not limited to polyamic acids, polyesters, polyamides, polyurethanes, silicones, proteins (such as modified gelatins), polypeptides, and polymers and copolymers based on ethylenically unsaturated polymerizable monomers such as acrylates, methacrylates, acrylamides, methacrylamides, vinyl ethers, vinyl esters, maleic acid/anhydride, itaconic acid/anhydride, styrenics, acrylonitrile, and olefins such as butadiene, isoprene, propylene, and ethylene. The starting polymers can contain more than one type of carboxylic acid-containing monomer. Certain monomers, such as maleic acid/anhydride and itaconic acid/anhydride may contain more than one carboxylic acid unit.
Preferably, the polymers are represented by the following Structure I or II: 
wherein the carboxylate-containing recurring units are linked directly (r is 0) within the polymer backbone in the recurring units identified as xe2x80x9cAxe2x80x9d units, or are connected by linking spacer units (r is 1) identified as xe2x80x9cWxe2x80x9d in Structures I and II above. This spacer unit can be any divalent aliphatic, alicyclic or aromatic group that does not adversely affect the polymer""s heat-sensitivity. For example, xe2x80x9cWxe2x80x9d can be a substituted or unsubstituted alkylene group having 1 to 16 carbon atoms (such as methylene, ethylene, isopropylene, n-propylene and n-butylene), a substituted or unsubstituted arylene group having 6 to 10 carbon atoms in the arylene ring (such as m- or p-phenylene and naphthylenes), substituted or unsubstituted combinations of alkylene and arylene groups (such arylenealkylene, arylenealkylenearylene and alkylenearylenealkylene groups), and substituted or unsubstituted N-containing heterocyclic groups. Any of these defined groups can be connected in a chain with one or more amino, carbonamido, oxy, thio, amido, oxycarbonyl, aminocarbonyl, alkoxycarbonyl, alkanoyloxy, alkanoylamino or alkaminocarbonyl groups. Particularly useful xe2x80x9cWxe2x80x9d spacers contains an ester or amide connected to an alkylene group or arylene group (as defined above), such as when the ester and amide groups are directed bonded to xe2x80x9cAxe2x80x9d.
While xe2x80x9crxe2x80x9d can be 0 or 1 in Structure I, preferably r is 0.
Preferably, A represents recurring units derived from ethylenically unsaturated polymerizable acrylates, methacrylates, acrylamides, methacrylamides, maleic acid or anhydride, or itaconic acid or anhydride.
Additional monomers (non-carboxylate monomers) that provide the recurring units represented by xe2x80x9cBxe2x80x9d in Structure I above include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable comonomers that may provide desired physical or chemical properties or which provide crosslinkable functionalities. One or more xe2x80x9cBxe2x80x9d monomers may be used to provide these recurring units, including but not limited to, acrylates, methacrylates, styrene and its derivatives, acrylamides, methacrylamides, olefins, vinyl halides, vinyl ethers, and any monomers (or precursor monomers) that contain carboxy groups (that do not have spiro-quaternary ammonium counterions).
The quaternary ammonium counterions in the polymers can include any compounds that comprise a tetracoordinate nitrogen positioned at the vertex of two intersecting ring systems. Preferably, these ring systems are represented by X, Y, and Yxe2x80x2 in Structures I and II taken with the illustrated nitrogen atoms.
Y and Yxe2x80x2 independently comprise any combination of carbon, nitrogen, oxygen, sulfur, phosphorous or selenium atoms sufficient to complete one or more heterocyclic ring systems with the illustrated nitrogen atom, each ring system independently having at least three atoms. Preferably, Y and Yxe2x80x2 independently complete ring systems with the illustrated nitrogen atom that can have from 3 to 9 atoms (carbon and heteroatoms). Such ring systems include, but are not limited to aziridine, azetidine, morpholine, piperidine, oxapyrrolidine, pyrrolidine, carbazole, indoline, and isoindoline rings. Additionally, the ring systems can contain one or more double bonds or may themselves be units in a more complex fused or compound ring system.
In Structure II, X represents at least two additional carbon, nitrogen, oxygen or sulfur atoms that together with the two illustrated nitrogen atoms, complete at least a 4-membered heterocyclic ring system located between the ring systems defined by Y and Yxe2x80x2. The types of ring systems that define X can be similar to those defined by Y and Yxe2x80x2 except that they have at least four valencies available for bonding to the two illustrated nitrogen atoms.
The individual atoms composing X, Y, and Yxe2x80x2 can contain substituents sufficient to fulfill the valency requirements. These substituents include independently substituted or unsubstituted cyclic, branched, or linear alkyl groups having 1 to 20 carbon atoms [such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, hydroxyethyl, 2-propanonyl, ethoxycarbonymethyl, benzyl, substituted benzyl (such as 4-methoxybenzyl, o-bromobenzyl, and p-trifluoromethylbenzyl), and cyanoalkyl], or substituted or unsubstituted aryl groups having 6 to 14 carbon atoms in the carbocyclic ring (such as phenyl, naphthyl, xylyl, p-methoxyphenyl, p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl, p-N,N-dimethylaminophenyl, methoxycarbonylphenyl and cyanophenyl). Other useful substituents for these various groups would be readily apparent to one skilled in the art, and any combinations of the expressly described substituents are also contemplated.
Alternatively, multi-cationic ionic species containing more than one quaternary ammonium unit covalently bonded together and having charges greater than +1 (for example +2 for diammonium ions, and +3 for triammonium ions) may be used in these novel polymers.
Also in Structure I, n represents from about 1 to 100 mol % (preferably from about 50 to 100 mol %) and m represents 0 to about 99 mol % (preferably from 0 to about 50 mol %).
While Structure I could be interpreted to show polymers comprised of only two types of recurring units, it is intended to include terpolymers and other polymers derived from more than two ethylenically unsaturated polymerizable monomers.
The spiro-ammonium ions of the present invention can be readily prepared using modifications of well known synthetic methods that are widely used for the synthesis of quaternary ammonium salts. These methods are described in many basic synthetic textbooks including March, Advanced Organic Chemistry, 3rd Ed., John Wiley and Sons, New York, 1985. Such methods will be obvious to one skilled in organic synthesis. One particularly useful synthetic method involves the reaction of a cyclic, secondary amine with a compound containing two leaving groups (such as halide or sulfonate esters) that are positioned in such a way that makes ring closure a facile pathway. The reaction of ammonia with two equivalents of such a compound will yield a symmetric spiro-ammonium salt.
The parent backbone polymers of this invention can be readily prepared using many methods that will be obvious to one skilled in the art. Many carboxylic acid or anhydride-containing polymers are commercially available. Others can be readily synthesized using preparative techniques that will be obvious to one skilled in the art.
The carboxylic acid or anhydride-containing polymers can be converted to the desired quaternary ammonium carboxylate salt by a variety of methods including, but not necessarily limited to:
1) the reaction of a carboxylic acid or acid anhydride-containing polymer with the hydroxide salt of the desired quaternary spiro-ammonium ion,
2) the use of ion exchange resin containing the desired quaternary spiro-ammonium ion,
3) the addition of the desired spiro-ammonium ion to a solution of the carboxylic acid-containing polymer or a salt thereof followed by dialysis,
4) the addition of a volatile acid salt of the desired spiro-ammonium ion (such as an acetate or formate salt) to a carboxylic acid-containing polymer followed by evaporation of the volatile component upon drying,
5) electrochemical ion exchange techniques,
6) the polymerization of monomers containing the desired spiro-ammonium carboxylate units, and
7) the combination of a specific salt of the carboxylic acid-containing polymer and a specific spiro-ammonium salt, both chosen such that the undesired counterions will form an insoluble ionic compound in a chosen solvent and can be removed after precipitate.